1. Field of the Invention
The present invention relates to a cavity shaped package having a recessed portion for accommodating electronic parts such as an integrated circuit, hybrid circuits and so on, in particular, a method of forming a package for electronic parts which is made from a metal plate.
2. Description of Related Art
Through the progress of information devices such as personal computers and so on in recent years, electronic parts such as semiconductor IC circuits or hybrid circuits have been advanced into higher density and higher processing speed, and the information devices using these electronic parts are becoming more precise and more miniaturized. On the other hands, higher density and higher processing speed of these electronic parts increases the amount of heat generated therein. Thus, heat sink means having high efficiency of heat radiation is required. However, the problem is that such miniaturized devices incorporating electronic parts cannot secure enough space for heat sink means that have necessary efficiency of heat radiation due to the miniaturization thereof.
In order to solve the problem, a cavity type package comprising a metal plate called xe2x80x9cstiffenerxe2x80x9d, which has a window or a recess at the center for accommodating electronic parts, has been occurred. The package allows the amount of heat generated from the electronic parts to be evacuated by the metal plate of the stiffener. Such package has in one body both functions as a heat spreader and as a stiffener for reinforcing the package. The package made from a metal plate will be an alternative to a conventional package, for example, made from ceramics since the former is superior to the latter in the heat sink function,
FIGS. 7 and 8 show a package of a semiconductor IC circuit as an example applying abovementioned package. The raw materials of the package 1 is selected from one of metal materials, which has a high thermal conductivity required as a heat spreader and for which plastically deforming processing is applicable, such as cupper alloy stainless steel, aluminum and so on.
An approximately rectangular recess portion 2 having a bottom portion 1c with a predetermined thickness is formed at the center of the surface on one side 1a of the package 1. That is, this package 1 is formed in a cavity shape as a whole. A wiring board 4, which is a TAB or flexible printed board, a usual pointed board, etc., is stuck and secured to the one side 1a of the package 1 by means of an adhesive. The wiring board 4 may be a layer on which a wiring circuit is formed and by which the wiring circuit is insulated from the package 1. An opening 4a having an approximately same size as the recess portion 2 is formed in this wiring board 4. A terminal portion 4t having a number of terminals is formed by printed wiring on the periphery of this opening 4a with a wire width and a pitch of about 37 nm. The terminals of the terminal portion 4t are electrically connected by printed wiring to externally connecting terminals (not indicated in the figures), which are formed on an outer periphery of the wiring board 4.
Further a chip 5 of electronic parts such as a semiconductor integrated circuit is accommodated in the recess portion 2 formed on the one side 1a of the package 1. The chip 5 is stuck to the bottom portion 1c of the recess portion 2 by means of an adhesive in a state where a surface of the chip 5 and that of the bottom portion 1c are tightly fitted. What is important in this sticking using an adhesive is that no bubbles should be produced between the chip 5 and the bottom portion 1c of the recess portion 2. This is because the bubbles are expanded and the chip 5 may be thereby peeled off in case where heat is produced during working of the electronic parts in the chip 5 and in case where heat is subjected to the package 1 during its assembling to an apparatus. Thus, the bottom portion 1c of the recess portion 2 should be formed in such an evenness that warp is smaller than 30 nm.
On the upper surface of the chip 5 there are disposed a number of terminals 5t with a same wire width and a same pitch as those in the terminal portion 4t formed on the wiring board 4. Each of the terminals 5t of the chip 5 and each terminal of the terminal portion 4t of the wiring board 4 are electrically connected by bonding wires 7, as indicated in FIG. 7. Further sealing agent 8 is injected into the recess portion 2 of the package 1 to seal the chip 5 and the bonding wires 7.
When the electronic parts such as an integrated circuit packed in the package 1 as described above is mounted onto a circuit board of an electronic apparatus not indicated in the figure, solder balls 9 are placed on the externally connecting terminals formed on an outer periphery of the wiring board 4 and they are melted by heating in a state where the package 1 is provisionary fixed at predetermined position on the circuit board of the electronic apparatus. In this way the electronic parts packed in the package 1 and the circuit board of the electronic apparatus are connected electrically via the wiring board 4 without any damages to the package 1 and its electronic parts due to expanding the bubbles between the chip 5 and the bottom portion 1c. 
On the other hand, when a semiconductor IC circuit is exothermic in operating, the package 1 itself has a function of heat spreader, thereby, heat is conducted the package 1 and is released.
In a forming method of above constituted cavity type package, for example, there is a method that a concave part is pressing processed using a pressing punch by press machine or by chemically etching process, a concave part is formed in a metal plate so that keep a bottom of a thin plate.
However, in a pressing process by press machine metal part of volume of the recess part is pushed into a bottom part and periphery. The periphery metal part is curled, then the flatness of the part is not able to correct state. Thereby, a package acquired necessity flatness has a fatal problem. The chemically etching process takes long time, so there are troublesome that the method is not suited in mass production and takes more cost in necessity. Moreover, precision by limiting to control etching process is worth, there is troublesome that practical use has a limit.
Inventors have proposed in U.S. Pat. No. 6,145,365 a preferred method for forming above mentioned type package made from a metal plate. FIGS. 9(A) to 9(D) shows the proposed method in summary. The method will be described herein below.
FIG. 9(A) shows a metal plate 100 that is positioned with respect to a die 104 of a press machine. FIG. 9(B) shows a pressing process, where the metal plate 100 is pressed recessed by a punch 105 of the press machine onto one side surface 100a so that the recessed part 102 is formed. At the same time, a protruding part 103 is formed in ledge shape on another side surface 100b of the metal plate 100.
FIGS. 9(C) and 9(D) show cutting process, wherein a cutter 106 shaves off the protruding part 103 formed on the surface 100b so as to form a thin bottom 100c and to make the surface 100b flat. In this cutting process, to avoid displacement of the bottom plate 100c in the direction to the recessed part 102, the bottom plate 100c is pressivelly supported by a supporting tool 107. According to the abovementioned processes, the recessed part 102 for accommodating a chip of electronic parts is formed in the metal plate 100, which has a bottom plate 100c having a predetermined comparatively thin thickness as the bottom of the recessed part 102. Thereby, the recessed part 102 of predetermined depth is formed by pressing and cutting without any damages to the bottom plate 100c. 
In the above methods, it is possible to form the recessed part 102 having a predetermined shape only on one side of the metal plate without giving the metal plate any remarkable stress or changing the composition of the metal plate because metal corresponding to the recessed part 102 is displaced to the protruding part 103 on the other side of the metal plate by plastically deforming processing by means of the press. However, in the cutting process, it is inevitable that the cutter 106 pulls the metal of the protruding part 103 around the cutting line by its movement. Therefore, the metal plate suffers a stress, especially at and around the bottom part 100c. 
Further, into the portion where the cutting by the cutter 106 ends, shown in left side on FIG. 9(D), a stress of pressure by the movement of the cutter 106 is concentrated. Such stresses provided by cutting process of the protruding part 103 lead to the problems that the dimensions of the package may change over time and that a time-varying warp may occur. Thereby, as mentioned above, in a package of electronic parts that requires such evenness that warp within 30 nm, the time-varying changes of dimensions and evenness become a fatal defect.
In order to solve such problems, the present invention, a package for accommodating in a recessed part formed one face of a metal plate, in package forming method for electronic parts formed cavity shaped which has thinner bottom part than the metal plate characterized in that, the package is formed the recessed part by pressing press machine or the like from one face of the metal plate and protruding part formed bulgingly in another face of the metal plate, by forming of the recessed part is cut by cutting tool dividedly in more than one and a cutting direction is differed in alternately facing to each other, so stress by cutting is almost cancelled.
Moreover, in the present invention, pressing process for pressing recessed part from one face of the metal plate and cutting process for cutting protruding part of another face of the metal plate are performed repeatedly more than one, and forming in stepwise desired depth of recessed and protruding parts in every forming frequencies a cutting direction may be differed in alternately facing to each other.
Moreover, in the present invention, pressing recessed part of predetermined depth in a face of the metal plate, protruding part of another face (opposite site face) of the metal plate is formed, in every cutting the protruding part dividedly in stepwise, a cutting direction may be differed in alternately facing to each other.
Moreover, in the present invention, when protruding part of another face of the metal plate is formed in every cutting the protruding part dividedly in stepwise, number of times of a cutting direction in alternately facing to each other may be the same.
Furthermore, in the present invention, predetermined recessed part which is shallower than thickness size of the metal plate from a face of the metal plate and the protruding part which is a little small than the recessed part in another face of the metal plate may be formed in every cutting the protruding part dividedly in stepwise, a cutting direction may be differed in alternately facing to each other.